1. Field of the Invention
The present invention relates generally to charged particle detectors and more particularly to single chamber ionization detectors applicable to combustion product and smoke detectors.
2. Background of the Prior Art
Ionization smoke detectors utilize a radioactive source to provide charged ions in a sensing chamber having an electric field. When the charged ions of the radioactive source are placed in an electric field, the positive ions migrate to the negative electrode of the field while the negative ions migrate to the corresponding positive electrode. The current generated by the emission of the ions from the source is extremely small, generally in the order of 10.sup.-11 amps. As the voltage across the electrical field is increased, there is a corresponding increase in the amount of current. However, a saturation current is reached at a specific voltage which is termed Saturation Voltage. Under normal conditions, the current is a function of the following factors: (1) ion mobility, (2) ion density per unit volume, (3) electric field intensity, (4) geometry of the chamber, and (5) the rate of source ion emissions (i.e., ions per unit volume per unit time).
A conventional smoke detector 100 operates, by reference to FIG. 1, as follows. The smoke detector 100 is mounted to the ceiling of room 110. When aerosols 120, generated by combustion of material 130 enter the chamber of detector 100, the aerosols 120 will deposit upon the ions. Generally, the aerosols are many thousand times larger than the emitted ions so that a marked decrease in the mobility of the combined ions and aerosols result in increased recombination (i.e., the combination of attraction of the negative ion with the positive ion) so that the current is correspondingly reduced. Conventionally, the change in current is detected as a voltage by a field-effect transistor which in turn drives an alarm device. The best operation range for the current flowing in the electric field is at a voltage that is substantially mid-range between the saturation current and zero current. This range is most sensitive to the presence of aerosols.
A discussion of commercially available ionization-type smoke detectors is found in the October, 1976 issue of Consumer Reports, pages 555-559.
Some prior art smoke detectors use only a single ionization chamber. A single ionization chamber device is shown in FIG. 2 to comprise a battery 200 connected in series with resistor 202 and the ionization chamber 204. A field-effect transistor 206 is interconnected across the resistor 202 and chamber 204 so that the gate of the field-effect transistor 206 is connected between the chamber 204 and the resistor 202, and the source and drain of the transistor 206 are connected across battery 200. An output voltage E.sub.0 is generated across resistor 208 which is interconnected between the drain and the negative side of the battery 200. In FIG. 3 the output 300 generated at E.sub.0 is shown before and after smoke entry in the chamber 204. Curve 300 is the output characteristic of the ionization chamber with no smoke while curve 302 is the output characteristic when smoke is present. Curve 304 represents the I-V characteristic for resistor 202. The disadvantages with the single chamber approach is that the resistor 202 is large being about 10.sup.11 ohms, is expensive to manufacture and is subject to leakage through contamination. Furthermore, variations in the radioactive source 210 located with chamber 204 causes variation in operating point and as a result sensitivity in chamber operation. Furthermore, if used, the sampling circuitry is complex and costly. Also, resistor 202 does not compensate for changes in humidity, air pressure, and temperature. Finally, calibration in adjustment is difficult since the sensitivity and stability is directly affected by source contamination such as dirt, etc. in the direction of the alarm. A prior art patent disclosing a single chamber device has been issued to McMillin et. al., Mar. 23, 1976, as U.S. Pat. No. 3,946,374.
A second type of ionization smoke detector uses two ionization chambers, one example being shown in FIG. 4. A battery 400 is connected in parallel across the dual chamber configuration 402. The upper chamber 404 is termed the "Reference Chamber" and that chamber is in a saturated current condition. The second chamber 406 is termed the "Sensing Chamber" and is in the unsaturated condition at the optimum operating point as previously discussed. The field-effect transistor 408 has its gate interconnected at the juncture 410 between the two chambers 404 and 406. A voltage E.sub.0 is developed across resistor 412. The operating characteristics for the two chamber detector is shown in FIG. 5. The Reference Chamber 404 with output curve 500 is shown to be in saturation condition while the Sensing Chamber 406 with output curve 510 is shown to be at the optimum operating point 520. When smoke enters chamber 406, the output voltage E.sub.0 drops to the curve 530. The signal voltage is shown as .DELTA.V. The use of the two chambers 404 and 406 eliminates many of the problems associated with the single chamber described above. Unfortunately, two radioactive sources are required with the result being a significantly higher manufacturing cost. Furthermore, the two radioactive sources must be matched since if a mismatch results, difficulty in adjustments and calibration occurs. Dust or chemical contaminants on either source can also cause an increase or decrease in sensitivity, depending upon which source is contaminated.
The following prior art patents represent variations of smoke detectors using two ionization chambers:
Lampert U.S. Pat. No. 3,710,110--Jan. 9, 1973, PA1 Scheidweiler U.S. Pat. No. 3,714,614--Jan. 30, 1973, PA1 Lehsten U.S. Pat. No. 3,903,419--Sept. 2, 1975, PA1 Scheidweiler et al U.S. Pat. No. 3,909,813--Sept. 30, 1975, PA1 Eguchi U.S. Pat. No. 3,909,814--Sept. 30, 1975, PA1 Emerson et al U.S. Pat. No. 3,952,294--Apr. 20, 1976, PA1 Tipton et al U.S. Pat. No. 3,959,788--May 25, 1976, PA1 Adachi et al U.S. Pat. No. 3,964,036--June 15, 1976. PA1 Lecuier U.S. Pat. No. 3,922,655--Nov. 25, 1975, PA1 Horvath et al U.S. Pat. No. 3,922,656--Nov. 25, 1975, PA1 Hurd U.S. Pat. No. 3,930,247--Dec. 30, 1975, PA1 Muller-Girard et al U.S. Pat. No. 3,936,814--Feb. 3, 1976, PA1 Gacoby U.S. Pat. No. 3,938,115--Feb. 11, 1976, PA1 Rayl et al U.S. Pat. No. 3,949,390--Apr. 6, 1976, PA1 Campman U.S. Pat. No. 3,950,739--Apr. 13, 1976.
Other types of smoke detector prior art devices are disclosed in the following patents:
One prior art approach is disclosed in the patent issued to Sasaki on Sept. 19, 1972, as U.S. Pat. No. 3,693,009. This approach utilizes a single ionization chamber, a pair of spaced electrodes in the chamber, and a grid electrode between the chamber. A potentional is applied between the facing electrodes and a voltage amplifier is connected between the grid and one of the electrodes to detect potential changes. The Sasaki approach utilizes the region between the first electrode and the grid as an internal chamber and the region between the first electrode and the facing electrode as the second external chamber. The Sasaki approach utilizes a direct current battery to bias the two facing plates so that a substantially linear voltage gradient is provided between the facing electrodes. The two facing electrodes are supported appropriately and smoke is directed therebetween upon the event of combustion. The presence of smoke in the external chamber causes a non-linear voltage gradient to exist between the first and second electrodes. The Sasaki device, however, while advantageously eliminating one of the two ionization chambers does not define the chamber response to pressure, temperature and humidity change. If the chamber electrodes are longer than the ion path, an increase in pressure causes an increase in ion collisions with neutral molecules thereby causing increased recombination and less ionization current at the electrodes. This tendency can be compensated by making the collector plate (electrode spacings) shorter than the distance of the ion path. Sasaki simply does not geometrically define the chamber. Furthermore, Sasaki discusses a "space charge limiting effect" due to ion recombination.
In "Ionization Dual-Zone Static Detector Having Single Radioactive Source," U.S. application Ser. No. 544,818, filed on Jan. 28, 1975, now U.S. Pat. No. 4,044,263, the inventor disclosed an ionization detector also including a single radioactive source having a small volume reference zone and a large volume signal zone set forth in a single ionization chamber. In this approach, a first electrode is preferably unitary in construction with the source of radiation. A second electrode either may be adjacent the walls of the housing or may be formed by the housing itself. A signal electrode is disclosed to extend axially through the axis of the housing disposed above the radioactive source. The reference zone is formed between the signal electrode and the radioactive source while the signal zone is defined by the large space separating the signal electrode and the second electrode or housing. A cylindrical housing is specifically disclosed wherein the height h would correspond to the point of maximum ionization from the point source. While this approach represents a vast improvement over the approach taught by Sasaki, the effect of change in pressures is simply not compensated for.
The importance of pressure, humidity and geometry on the operation of a detector is mathematically analyzed and discussed in "Ionization-Type Smoke Detectors," Simon and Rork, Rev.-Sci. Instrum., Vol. 47, No. 1, Jan. 1975, pgs. 74-80, and in "Analysis of an Ionization Chamber-Aerosol and Combustion Sensing System," Klein, Transactions of Instrumentation and Measurement, Vol. IM-20, No. 1, Feb. 1971, pgs. 33-37.
The following invention is a dramatic improvement over the above prior art improvements as will be discussed and brought out below.